The present invention is directed to a method for indicating ventilation efficacies occurring in the lungs of a subject and the volumetric portions of the lungs possessing differing ventilation efficacies. Such information is useful in determining the uniformity or homogeneity, or lack thereof, in the ventilation occurring in the subject's lungs. The method may also be used to determine the volume of the lungs.
When breathing gases are inspired by a subject and pass from the patient airways into the lungs, the inspired breathing gases travel in the direction of the least flow impediment, of highest lung elasticity, and of smallest gas flow resistance. The lungs are always inhomogeneous with respect to these factors, and therefore the distribution of the inspiration gas flow into, or ventilation of, the lungs is uneven.
When a given volume of inspired breathing gas VA flows into a lung compartment of volume V, the ventilation of that compartment is defined as VA/V. The smaller the inspired gas volume VA entering a given compartment, compared to the compartment volume V, i.e. the lower the VA/V value, the smaller will be the amount of blood flow for which that compartment can provide adequate oxygenation and carbon dioxide removal and, thus, the less effective, or efficacious, this lung compartment will be in these basic functions of respiration. In mathematical treatment and analysis of the lung, the categorizations of lung characteristics employed in/resulting from such treatments are often expressed as “compartments,” as the term is used above. It is to be understood that such compartmental descriptions are analytical concepts that do not necessarily have any direct correlation to anatomical portions of the lungs, such as lobes. The gases in a given compartment are considered perfectly mixed.
When the lungs are healthy, lung ventilation is sufficiently homogeneous throughout the lung for the physiological gas transfer requirements of the subject. However, the situation changes significantly with various lung diseases that may cause the ventilation VA/V in portions of the lungs to be as small as 1/100 or even 1/1000. When there is little, or no, ventilation in a portion of the lungs, but blood flow or perfusion is present, a “shunt” is said to exist in the lung in which gas/blood transfer is lessened or eliminated. Venous blood, i.e. unoxygenated blood from the shunt, becomes mixed with oxygenated and CO2 depleted blood from properly functioning parts of the lungs. This reduces the total blood oxygenation/CO2 elimination carried out in the lungs.
During inspiration, a lung compartment fills with the inspired gas volume VA and a practically equal volume of breathing gases is removed during expiration, although normally O2 uptake into blood is a little larger than CO2 release from blood. Assume for simplicity that a lung compartment has a concentration Fo of a particular gas, such as nitrogen, that is not readily exchanged with blood. For a steady state of breathing, the concentration in the lungs will be the same as the concentration in the inspired breathing gases. The concentration F of the gas in a compartment after inspiration has occurred is                     F        =                              F            0                    ·                      V                          V              +                              V                A                                                                        (        1        )            The term V/(VA+V) is a dilution factor, or the ratio of the new compartment concentration F after inspiration of volume VA to the concentration Fo in the compartment before ventilation. The more the compartment is ventilated, the lower the new compartment concentration F will be and the smaller the number of breaths needed to deplete the compartment of the gas, and vice versa. For explanatory purposes, a gas, such as nitrogen, is termed herein an “inert” gas, although it is recognized there may be some transfer to/from the blood in the lungs depending on the relative amounts of the gas in the inspired breathing gases and in the blood. However, the exchange occurring with the blood is negligible compared to that occurring from ventilation.
As the subject is usually ventilated by the aid of a mechanical ventilator, the inspired breathing volume VA may be determined from the ventilator. Or the volume may be directly measured as it is inspired by the patient.
Lung volume V can be measured by an inert gas elimination technique. In this method the lungs are first ventilated with a breathing gas mixture of known composition and containing an inert gas. At a steady state condition, i.e. the state where the expired gas composition is unchanged from breath to breath, a sudden change in the inspired gas mixture inert gas content is initiated. The expired inert gas concentrations and volumes are analyzed for the breaths following the sudden change. Lung volume V is then calculated from this data as:                               Lung          ⁢                                          ⁢          Volume          ⁢                                          ⁢          V                =                                            ∑              breaths                        ⁢                          Δ              ⁢                                                          ⁢                              V                ig                                                          F            -                          F              0                                                          (        2        )            where (ΔVig) is the change in expired inert gas volume in the lungs for a single, given breath, F is the inert gas concentration of the lungs after the latest breath, and F0 is the steady state inert gas concentration before the inert gas change was initiated. The lung inert gas concentrations F may be conveniently determined as the end tidal breathing gas concentrations of the subject. The lung volume V measured by Equation 2 is the physical, anatomical volume of the lungs.
Typically the inert gas used for the analysis is nitrogen (N2) as described above, but also helium (He), sulfur hexafluoride (SF6), and fluoropropane (US 2002/0052560) may be used.
The above volume measurement technique, carried out for a number of breaths, forms a data series for the inert gas concentrations F in the lungs and for single breath inert gas volume changes ΔVig. These data series have also been used to analyze the homogeneity, or inhomogeneity, of lung ventilation. Some indices of lung homogeneity/inhomogeneity are based on analysis of the rate of change of inert gas concentration as related to gas turnovers. The term “turnover” describes the relationship between ventilation volume VA and lung volume. For example, if a single compartment lung has a volume of 3000 ml and the ventilation volume VA is 500 ml, a turnover is six breaths. Other indices follow the rate of change of total expired inert gas volume.
In each of these methods the resulting index is a number having an abstract scale, which is not related to clinically important, self-explanatory terms, such as lung volume or ventilation efficacy. Therefore, although these indices may have good correlation with ventilation inhomogeneity, they have not gained clinical acceptance.
Another problem with these methods is that in real multi-compartment lungs, the data series for the F and ΔVig values often represent different change response patterns, and when only one of these factors is considered, some part of the information will be lost.
The inhomogeneity of the lung ventilation is as important a parameter as the lung volume itself since in various diseases, part of the lung volume may have insufficient ventilation for the needed of blood/gas transfer. Therefore, it is important to know not only the lung volume, but also how well this volume is utilized, i.e. ventilated. Knowing this information will give a clinician new tools for optimizing ventilation and utilizing other patient therapies by giving a direct indication of the response of the subject to the measures taken to improve the lung function. It is further desirable that presentation of the information be self-explanatory to the clinician